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You’ve heard the rumours and seen the predictions, but what exactly does a taller gear ratio mean for Bathurst this year?

Reports suggest Greg Murphy’s Lap of the Gods will be smashed and top speeds will reach as much as 310km/h. Hell, at this rate, Lowndsey and J-Dub might find themselves launched into the atmosphere with a strong gust of wind.

So, to make sense of it all, we went to a man who puts the different into differential – Car No.888 engineer Jeromy “JJ” Moore.

RBRA: First thing’s first, JJ, what exactly does a taller gear ratio/differential mean?

JJ: Basically, a taller ratio means a longer gear, which in turn means a little less acceleration but a higher top-end speed. It changes every gear to make it a little bit taller. So it’ll be a bit harder to get off the line. Some corners, like The Cutting, the engine will be even lower in revs again so it will likely make it a little more difficult coming out of there. There will have to be more emphasis on engine mapping to make sure the engine is tuned correctly down low. Every gear will be ramped up so that means you’ll lose a bit of time in certain parts of the track but that should be made up on the straight. That means the laps at best would be marginally faster if the engine mapping isn’t too bad down low. It’ll probably be a little more difficult to drive in the slow parts but overall with conditions in our favour, it should be faster.

RBRA: How much faster?

JJ: The taller gear in itself will probably only take a couple of tenths off Conrod Straight at best with theoretically another 10km in top speed to the rev limiter– so around 310km/h. But at most, I reckon maybe a few extra km/h as there likely won’t be enough time to reach the limiter. The wind and air temp will also have a massive effect. Where it will be of benefit is if you’re slipstreaming and you pull out behind someone, rather than hitting the limiter and not being able to get past them, the ratio change should allow you to go faster and make for more passing.

RBRA: What will cause V8s to act and implement the change?

JJ: It’s very interesting. We’re going there with the same effective ratio we had last year and V8s will monitor it in practice to see if we are hitting the rev limiter too much. We typically hit the rev-limiter for about 3-5 seconds down Conrod Straight which means we can’t go any faster than about 300km/h. And with the new aerodynamics and Car of the Future, there’s a thought that some cars might hit the limiter earlier than usual and possibly cause too much wear and tear to the gearbox.

RBRA: Is it likely to happen or just speculation? And how would it affect Lowndesy and J-Dub?

JJ: I don’t think we will hit it enough to warrant the change – because every team will have to change the gearing over the course of the weekend and that isn’t ideal. But for us, we don’t mind at all. We’d actually prefer the taller gear but I’m sure other teams who might be down on power wouldn’t prefer it. It’s probably not a good chance to get up but it’d definitely suit us if it did.

RBRA: Other than Red Bull Racing and the big factory teams, which manufacturer will gain the biggest advantage from a ratio shift?

JJ: Obviously the big three teams and BJR and GRM would get a gain, but I think maybe Erebus would have the most to gain in terms of using the power they have at their disposal. They have a big motor with lot of power – a taller ratio should allow them to use some of that. They’ll be very strong either way – it’ll only be a matter of reliability. But I wouldn’t be surprised to see a top-five finish from them whether we see the change or not. At our last test day they were on the rev-limiter harder than us and I expect they will be very strong in a straight line.

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